It happened in Toronto a few years back, a local goodwill store received two small oil paintings. a staff thought they might worth some money because the frames looked expensive and they did. it end up sold at art auction for ten of thousands dollar.
I used to go shopping at goodwill a lot. One time I found a framed etching but without glass had a price tag for $7 but right away I recognized it was from a well known Canadian artist because she gave a lecture on her works at the art school I attended. I brought it and had it re-frame and still own it.

I get frames and non-glare glass which I ordinarily cannot afford to buy at thrift stores not necessarily Goodwill. I see original art and am saddened thinking that my art may end up here in one of these stores someday after I'm dead. The I decided to destroy all my art before I die if it will not get sold at my pre-death estate auction.

Many collectors own my art hoping just like me but it may all end up in the dumpster. Thrift stores have a low price on everything "used" and donated for free to them.

I used to frequent Goodwill back when they actually had good stuff. I found an Edward S. Curtis photograph there several years ago that was appraised for $300 by a Seattle company, among other things... probably worth more now. However, once eBay happened, Goodwill started their own site, and now they put nothing but junk on their shelves. I keep waiting for them to taper off their net site and get back to putting good stuff in the stores again. Occasionally, though, something will slip through. You're right, Vincent, about the books! But I've found better ones at library sales, and for much cheaper.

Janine, I kept most of my thrift store art when I downsized. Most of it in the "shabby chic" roses style that's not so hot today, but I absolutely love it and just couldn't part with it. Sigh... :)

The days of finding steals at thrift stores and even garage sales is all but over. With the internet and shows like "Antiques Roadshow" too many people are fully aware of what their "junk" COULD be worth. Stores like Goodwill and Salvation Army here in Ontario go through everything that comes in with a fine tooth comb, sorting out potential valuables that can be auctioned off to bring in more money for their cause (and good for them for taking the time) and everything else goes to the retail stores. Every now and then something valuable slips through the cracks, but that is becoming rare indeed.

Most of what I see at these stores now, truly is another persons "junk".